In recent years, various types of flat panel display devices lighter and smaller than cathode ray tube (CRT) screens have been developed. Among the different types of flat panel display devices, active-matrix organic light-emitting diode (AMOLED) display devices utilize self-light-emitting organic light-emitting diodes (OLEDs) to display images, resulting in a shortened response time, less drive power consumption, and relatively better brightness and color purity characteristics. Thus, OLED display devices have become the focus of the next generation display devices.
A large AMOLED display device includes a plurality of pixels at intersections between scan lines and data lines. Each pixel includes an OLED and a pixel circuit that drives the OLED. The pixel circuit typically includes a switch transistor, a drive transistor and a capacitor. As the properties of the pixels in the AMOLED display device are adversely affected by differences between the drive transistors as well as by leakage currents in the switch transistors, the images displayed using these pixels suffer from low uniformity and consistency in quality.
FIG. 1 is a circuit diagram of a conventional pixel in an AMOLED display device. As shown in FIG. 1, the conventional pixel 10 in the AMOLED display device includes an OLED 14 and a pixel circuit 12 that is connected to a data line Dm and a scan control line Sn in order to enable control of the OLED 14.
Specifically, the pixel circuit 12 includes a second transistor M2 (i.e., a drive transistor) connected between a first power source ELVDD and an anode of the OLED 14, a first transistor M1 (i.e, a switch transistor) connected between a gate of the second transistor M2 and the data line Dm, and a capacitor Cs connected between the gate of the second transistor M2 and the first power source ELVDD, wherein a gate of the first transistor M1 is connected to the scan control line Sn. The OLED 14 has an anode connected to the pixel circuit 12, and the OLED 14 has a cathode connected to a second power source ELVSS. The OLED 14 emits light whose brightness corresponds to a current supplied by the pixel circuit 12.
When a scan signal is provided to the scan control line Sn, the pixel circuit 12 controls the magnitude of a current supplied to the OLED 14 in correspondence to a data signal provided to the data line Dm. Specifically, the gate of the first transistor M1 is connected to the scan control line Sn, with a source (or drain) of the first transistor M1 connected to the data line Dm and the drain (or source) of the first transistor M1 connected to a terminal of the capacitor Cs. When a scan control signal is provided to the first transistor M1 from the scan control line Sn, the first transistor M1 is turned on, and the data signal provided to the data line Dm is then supplied to the capacitor Cs. One terminal of the capacitor Cs is connected to the gate of the second transistor M2, and the other terminal is connected to the first power source ELVDD. A voltage corresponding to the data signal is applied to the capacitor Cs. As a result, the voltage corresponding to the data signal is stored in the capacitor Cs.
The gate of the second transistor M2 is connected to the one terminal of the capacitor Cs, a source of the second transistor M2 is connected to the first power source ELVDD and a drain of the second transistor M2 is connected to the anode of the OLED 14. The second transistor M2 controls a current flowing from the first power source ELVDD, through the OLED 14, toward the second power source ELVSS. This current corresponds to the voltage stored in the capacitor Cs.
The conventional pixel 10 controls the brightness of the OLED 14 by providing the OLED 14 with the current that corresponds to the voltage applied to the capacitor Cs and thereby displays an image with predetermined brightness. However, due to differences between threshold voltages of the second transistors M2 as well as leakage currents in the first transistors M1, it is a challenge for the conventional AMOLED display device to display an image with uniform brightness.
For example, due to the differences between threshold voltages of the second transistors M2 in different pixels, the application of the same gate drive voltage leads to different currents flowing through the OLEDs and hence their different brightness levels. Since the pixels produce non-uniformly bright light in response to the same data signal, an image with uniform brightness is hardly obtainable.